I'm pretty sure the shift really kicks in when Frank Miller took over writing DD in 1979 or 1980 or so. Miller really started pushing the Kingpin angle. Previously, the Kingpin had been both a DD and Spidey villain, but Miller made the Kingpin DD's archnemesis (along with Bullseye), Miller introduced Elektra and the whole ninja aspect, he was (I think) the first to get into DD first getting started with his vigilante career based on what happened with his father being pushed around by organized crime enforcers, etc.
You can see a lot of this in the first volume of collected Miller DD works, which, incidentally is...ok. It doesn't REALLY get good until Miller is given full writing control which doesn't happen until towards the end of the volume, and even then, it takes him a while to "find his voice" as distinct from what the other Marvel writers were doing at the time. So, for a while, it reads much more like a kidn of late-70s campy superhero comic. By the last two issues in the volume, though, you see the first kernels of what Miller's style will become which, in my opinion, culminates with the Born Again story arc (which I've been waiting for in film or -- better yet -- episodic TV format for just shy of 30 years...).
It's looking like this first season of DD is going to be heavily indebted to the Man Without Fear arc, although I expect it will expand on that further. If you haven't read any of these yet, DEFINITELY read The Man Without Fear, and try to grab a copy of Born Again. Miller writes it, but Dave Mazzuchelli's artwork is phenomenal and very, very cinematic, down to his use of lighting in various panels. It's practically a storyboarded screenplay.
